Stereophonic sound recording and reproduction



Feb. 24, 1942. G. HOLST ETAL I 2,273,866

STEREOPHONIC SOUND RECORDING AND REPRODUCTION Filed Sept. 27, 1958AMPLIFIER POW!!! AHPLIFIEI A NPLIF/ER l POWER. l

AIPLIFIER 6'! 49.1. 5s 6: AM. Pawn W ll L.. J AMPLIFIER 55 AMPLIFIER 55: 4

4 W as Patented Feb. 24, 1942 UNITED STATES PATENT OFFICE STEREOPHONICSOUND RECORDING AND REPRODUCTION Application September 27, 1938, SerialNo. 231,997

In Germany October 4, 1937 9 Claims.

The present invention relates to a method and apparatus for thestereophonic recording and reproduction of sound and to a sound recordfor use in such reproduction.

The term stereophonic reproduction of sound as used herein and in theclaims is to be understood to mean that type of reproduction in whichthe sound is reproduced by a plurality of reproducing devices in such amanner that a space efiect is obtained, i. e. so that it is possible tolocate a sound source in space even in j: the artificial reproduction.Thus the term stereophonic reproduction includes not only the wellknownreverberation effect, but also the directional effect which allows oneupon hearing a sound to localize its source and/or ascertain themovement thereof.

In sound recording it is well known to use for the above purpose, aplurality of specially-separated receiving devices whose vibrations aretransmitted through separate channels which are subsequently reproducedseparately by means of spacially-separated reproducing devices. It isalso well known to divide the frequency range of the sound vibrations tobe recorded and reproduced, into a plurality of frequency ranges toavoid distortion. However, when combining this measure with stereophonicrecording and reproduction, the apparatus becomes very complicated. Moreparticularly, if the frequency range is divided into n-parts and thereis stereophonic transmission in m-channels, it will be necessary to use12 times m transmitting and reproducing devices.

The object of our invention is to overcome the above difficulties and tosimplify such methods of recording and reproduction as well as theapparatus used.

A further object of our invention is to provide a new type of soundrecord for use in such types of reproduction.

We have found that the sound vibrations in the lowest-frequency rangeare not essential for the stereophonic effect. More particularly thesesound vibrations have a low frequency and large wave length, and thushave only a very slight directional effect and are not essential for 10-cating the sound source.

In accordance with the invention, we supply the sound vibrations in thehigher frequency range of each recording device, to separate channels,and supply the vibrations in the lower frequency range of these devicesto a single channel. The higher frequency ranges, are reproduced byreproducing devices which are separated both acoustically and spacially,whereas the vibrations of the lowest frequency range are reproduced by acommon reproducing device.

The term "channel" as used herein and in the claims is to be understoodto include both a recording of the sound vibrations such as a soundtrack on a carrier, and a path in which the electrical currentvariations corresponding to the sound vibrations can flow, for instance,a carrier wave or a transmission line.

In one form of the invention in which the vibrations are recorded, werecord the high-frequency ranges in separate sound tracks on a carrierand separately reproduce these tracks with specially-separatedreproducing devices, whereas we record the lowest frequency range in acommon sound track and reproduce this latter track by a commonreproducing device.

The upper limit of the lowest frequency range, which is consequentlycommon to all recording and reproducing devices, generally lies between200 and 300 vibrations per second. Below these frequencies the soundwaves have substantially no directional effect.

In order that the invention can be more clearly understood and readilycarried into effect, we shall describe the same in more detail withreference to the accompanying drawing in which:

Figure 1 is a schematic diagram of a stereophonic reproducing systemembodying the invention,

Fig. 2 is a schematic diagram of a stereophonic recording systemembodying the invention,

Fig. 3 is a schematic diagram of a stereophonic 1eproducing system forreproducing the record made-in system of Fig. 2, and

Fig. 4 is a perspective view on an enlarged scale of a stereophonicsound record according to the invention.

In the reproducing system shown in Figure 1 a sound carrier I 0 providedwith two stereophonically-recorded sound tracks H and I2 is reproducedby two similar reproducing devices, one including an objective l3 and aphoto-electric cell I5 and the other including an objective I 4 and aphoto-electric cell IS. The tracks H and I2 are produced in the usualmanner by the use of receivers which are spaced apart, and each trackincludes all the frequencies recorded.

The electric current variations produced by photo-electric cell I5 areamplified by a. preliminary amplifier IT and a power amplifier l9, andthe high frequency variations pass through a high pass filter 2| to areproducing device 25 and the lower frequencies pass through a low passfilter 22 to a reproducing device 21. Similarly the current variationsproduced by photo-electric cell l5 are amplified by a preliminaryamplifier l8 and a power amplifier 20, and the high frequency variationspass through a high pass filter 24 to a reproducing device 26 which isspaced from reproducing device 25, whereas the low frequency variationspass through a low pass filter 23 to the reproducing device 21.

The high pass filters 2i and 24 allow the passage of only oscillationsabove about 200 to 300 cycles per sec. and preferably about 250 whereasthe low pass filter 22 and 23 allow the passage of only oscillationswhose frequencies lie below this value.

Thus, with a system such as shown in Fig. 1, the stereophonic soundrecord, as regards the high-frequency range, is separately supplied tothe spacially-separated reproducing devices 25 and 26, whereas all thelow frequency oscillations of both tracks II and 12 are supplied to thesingle reproducing device 21.

In the drawing each of the reproducing devices 25, 26, and 21 consist ofonly a single loudspeaker. However, it should be well understood thatfor each of these devices it is possible to substitute a plurality ofloudspeakers which are connected in parallel acoustically. For instancereproducing device 21 may be replaced by a plurality of loudspeakerswhich are electrically interconnected to the feed conductor 15, or someof the speakers may be supplied directly from filter 22 and otherssupplied directly from filter 23. However, with the method according tothe invention, one reproducing device, i. e. a loudspeaker or aplurality thereof connected acoustically, can be dispensed with. If inaccordance with the prior art each frequency range were reproduced in aseparate manner acoustically, it would be necessary to use two of thereproducing devices 21.

In the recording system illustrated in Fig. 2, the sound to be recordedis picked up by two specially-separated sound receiving devices 28 and29 which together eifect the stereophonic recording. A shown each deviceconsists of a single microphone, but each may comprise a plurality ofelectrically-connected microphones which are connected in parallelacoustically.

The electrical current variations produced by device 28 are amplified ina preliminary amplifier 30, whereupon the high frequency variations passthrough a high pass filter 32, are amplified by a power amplifier 36,and applied to a recording device 39. The low frequency variationsleaving amplifier 30 pass through a low pass filter 33, are amplified bya power amplifier 31, and transmitted to a recording device 40. In asimilar manner, the current variations produced by device 29 areamplified by a preliminary amplifier 3|, filtered by low pass filter 34and high pass filter 35, the high frequencies being amplified by a poweramplifier 38 and transmitted to a third recording device 4|, and the lowfrequencies being amplified by amplifier 31 and passing to recordingdevice 40.

The recording may be effected in any of the well known manner, but asindicated it is effected mechanically by the method described in the U.S. Patent #l,9l9,116 toJames A. Miller. In this case each of therecording devices 39, 40 and 4! is provided with a cutting tool having aV shaped cutting edge which cuts a track in a carrier 45. As shown moreclearly in Figure 4 the carrier 45 comprises a cutting layer 16 oftransparent material and a thin covering layer 11 of opaque material,the tracks 42, 43 and 44 being produced by removing portions of layer11.

The filters 32 and 35 pass only frequencies above above 250 cycles persec. so that the two sound tracks 42 and 44 are stereophonic records ofthe high frequency range. However, the variations of the low frequencyrange are passed by the low pass filters 33 and 34, which have a cutoffat about 250 cycles per sec., and after bein amplified by the commonamplifier 31, are recorded by the single recording device. Thus, it isseen that the apparatus is considerably simplified because only a singlepower amplifier 31 and recording device are used for the low frequencyrange.

As shown in Fig. 4 the resulting sound carrier has only a single track43 for the low frequencies, and two stereophonic tracks 42 and 44 forthe high frequencies. In some cases, for instance for stereophonictransmission of sound in large rooms, it may be desirable to use apurality of recording groups. For instance, it may be desirable to usesix tracks in the high range and two tracks in the low range. In anycase, however, the invention makes it possible to reduce the number oftracks and thus to simplify the recording apparatus.

In the reproducing system of Fig. 3, the sound tracks 42, 43 and 44 ofthe carrier 45 are optically scanned by three objectives 52, 5| andrespectively to produce current variations by means of photo-electriccells 55, 54 and 53 respectively.

The reproducing is effected by two spaciallyseparated reproducingdevices 62 and 63. Device 62 comprises a high-frequency loudspeaker 64and a low-frequency loudspeaker 66, and device 63 comprise ahigh-frequency loudspeaker 65 and a low-frequency loudspeaker 61. Thecurrent variations produced by cell 55 are amplified by a preliminaryamplifier 58 and a power amplifier GI, and supplied to speaker 64, and

similarly the current variations produced by cell; 53 are amplified by apreliminary amplifier 55- and a power amplifier 59 and supplied tospeaker 65. The low-frequency current variations produced by cell 54 areamplified by a preliminary amplifier 51 and a power amplifier 60, andthe amplified current variations are supplied to both the speakers 66and 61.

Instead of reproducing the low frequency range by means of the speakersand 61, which are acoustically combined with the high-frequencyspeakers, it is of course possible to use a separate reproducing devicefor this purpose, as shown in Figure 1. In this case it is possible touse for the low frequencies a single speaker or a plurality of speakerswhich are separated acoustically from the high-frequency speakers.

Although Figures 2 and 3 illustrate recording and reproduction in whichthe sound is recorded in a carrier and then reproduced, it should beunderstood that our invention is not limited thereto. The invention alsoextends to a completely electrical system in which the vibrations aretransmitted completely by electric transmission, i. e. by wire orcarrier wave, from the recording point to the reproducing point.Furthermore, it is possible to divide the high frequencies into aplurality of bands and reproduce these by additional speakers.

While we have described the invention in connection with specificexamples and applications, we do not wish to be limited thereto butdesire the appended claims to be construed as broadly as is permissiblein view of the prior art.

That we claim is:

1. A method of sterophonically recording sound vibrations by means of aplurality of recording devices, comprising the steps of dividing thevibrations into a plurality of frequency ranges, transmitting inseparate channels the vibrations in the higher frequency ranges of eachrecording device, and transmitting in a single channel the vibrations inthe lowest frequency range of said devices.

2. A method of stereophonically recording sound vibrations by means of aplurality of recording devices comprising the steps of dividing thevibrations into a plurality of frequency ranges including a lowfrequency range whose upper limit lies between about 200 to 300 cyclesper sec., transmitting in separate channels the vibrations in the higherfrequency ranges of each recording device, and transmitting in a singlechannel the vibrations in the low frequency range.

3. A method of stereophonically reproducing sound vibrations by means ofseparate reproducing devices, comprising the steps of separating thevibrations into a plurality of frequency ranges, transmitting thevibrations of the higher frequency ranges to a plurality of said deviceswhich are separated acoustically and spaclally, and transmitting thevibrations of the lowest frequency to only one of said reproducingdevices.

4. A method of stereophonically reproducing sound vibrations by means ofa. plurality of separate reproducing devices, comprising the steps oftransmitting the vibrations by separate channels, separating thevibrations of each channel into a plurality of frequency rangesincluding a low frequency range having ,an upper limit between 200 to300 cycles per sec., transmitting the vibrations of the high frequencyrange to a plurality of said devices which are separated spacially andacoustically, and transmitting the vibrations of the low-frequency rangeto only one of said reproducing devices.

5. A method of stereophonically recording sound vibrations on a carriercomprising the steps of separating the vibrations into a plurality offrequency ranges including a low frequency range, recording thevibrations of the low frequency range in a single sound track, andrecording the remaining vibrations in a plurality of sound tracks.

6. A method of reproducing sound vibrations from a stereophonic soundrecord having a plurality of sound tracks, comprising the steps ofscanning the sound tracks, separating the current variations produced bythe scanning into a plurality of frequency ranges including a lowfrequency range, transmitting the low-frequency variations to a singlereproducing device, and transmitting the remaining frequencies to aplurality of acoustically-separated reproducing devices.

'7. A stereophonic sound record comprising a carrier provided with aplurality of highfrequency sound tracks and low-frequency sound tracks,the number of the low-frequency sound tracks being substantially lessthan the number of high-frequency sound tracks.

8. A stereophonic sound record comprising a carrier provided with aplurality of high-frequency sound tracks and a single low-frequencysound track.

9. A sterephonic sound recording system comprising a plurality of soundreceiving devices, filtering means for separating the current variationsof each device into a low-frequency range and a high-frequency range,means to apply the high-frequency variations produced by each device toindividual recording devices, and means to apply the low-frequencyvariations produced by all the devices to a single recording device.

GILLES HOLST. KORNELIS DE BOER.

